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Journal of Nanotechnology
Volume 2012 (2012), Article ID 101243, 6 pages
http://dx.doi.org/10.1155/2012/101243
Research Article

Raman Laser Polymerization of C60 Nanowhiskers

National Institute for Materials Science, Fullerene Engineering Group, 1-1, Namiki, Ibaraki, Tsukuba 305-0044, Japan

Received 14 July 2011; Revised 25 December 2011; Accepted 4 January 2012

Academic Editor: Junfeng Geng

Copyright © 2012 Ryoei Kato and Kun'ichi Miyazawa. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Photopolymerization of C60 nanowhiskers (C60NWs) was investigated by using a Raman spectrometer in air at room temperature, since the polymerized C60NWs are expected to exhibit a high mechanical strength and a thermal stability. Short C60NWs with a mean length of 4.4 𝜇m were synthesized by LLIP method (liquid-liquid interfacial precipitation method). The Ag(2) peak of C60NWs shifted to the lower wavenumbers with increasing the laser beam energy dose, and an energy dose more than about 1520 J/mm2 was found necessary to obtain the photopolymerized C60NWs. However, excessive energy doses at high-power densities increased the sample temperature and lead to the thermal decomposition of polymerized C60 molecules.

1. Introduction

C60 nanowhiskers (C60NWs) are the single crystal nanofibers composed of C60 molecules [1] and can be synthesized by a facile method called “LLIP method” [2]. C60NWs have a variety of applications and such as field-effect transistors (FETs) [3], solar cells [4], biosensors [5].

C60 molecules can be polymerized by electron beam irradiation [6]. Although as-grown C60NWs are composed of the C60 molecules that are weakly bonded via van der Waals forces [7], the C60NWs irradiated by electron beams showed the stronger thermal stability [8], the higher Young’s modulus [9] than pristine van der Waals C60 crystals. Hence, it is of great importance to study the polymerization of C60NWs in order to improve their mechanical and thermal properties.

Laser irradiation is a promising method to obtain the polymerized C60 molecules [7, 10]. We first showed the photopolymerization of C60NWs by using the Raman laser beam irradiation [7]. Rao et al. showed that the peak of Ag(2) pentagonal pinch mode of C60 shifts downward from 1469 cm−1 to 1459 cm−1 upon the photopolymerization [11], showing that the shift of Ag(2) peak is a good indicator for the polymerization of C60.

Alvarez-Zauco et al. studied the polymerization of C60 thin films in air by the ultraviolet (UV) laser irradiation as a function of laser energy dose (= fluence) from 10 to 50 mJ/cm2 in order to optimize the photopolymerization of C60 films [12]. Likewise, the laser energy dose for the photopolymerization of C60NWs should be optimized. Hence, the present study aims to reveal how the polymerization of C60NWs proceeds as a function of the laser beam energy dose.

2. Experimental

C60NWs were synthesized by a modified liquid-liquid interfacial precipitation method. Isopropyl alcohol (IPA) was gently poured into a toluene solution saturated with C60 (MTR Ltd. 99.5%) in a glass bottle to make a liquid-liquid interface, and then the solution was subjected to ultrasonication and stored in an incubator at 10°C to grow short C60NWs. The synthesized C60NWs were filtered and dried in vacuum at 100°C for 120 min. to remove the solvents. In the Raman spectrometry analyses, the C60NWs dispersed in ethyl alcohol were mounted on a slide glass and dried in air.

A Raman spectrometer (JASCO, NRS-3100) with a green laser of 532 nm excitation wavelength was used for the polymerization and structural analysis of C60NWs in air. The power of laser light illuminated onto the specimens was measured by using a silicon photodiode (S2281, Hamamatsu Photonics K.K.). The laser beam power density (𝐷) and the energy dose of excitation laser beams in the Raman spectroscopy were controlled by changing ND (Neutral Density) filters, the defocus value of objective lens, and the exposure time of laser beam. 𝐷 is defined by the following formula in this paper,𝐷mW/mm2=Thepoweroflaserbeam(mW)theareaoflaserbeamexposedonthesamplemm2.(1)

3. Results and Discussion

Figure 1 shows examples of scanning electron microscopy (SEM) images and the size distributions of the synthesized C60NWs with a mean length of 4.4 ± 2.7 μm and a mean diameter of 540 ± 161 nm. The distribution of aspect ratios (length/diameter) is also shown. Most of the C60NWs were found to possess the aspect ratios less than 15.

fig1
Figure 1: (a) SEM images, (b) length, (c) diameter, and (d) aspect ratio (length/diameter) distributions of the synthesized C60NWs.

The power of excitation laser beam can be changed by selecting ND filters. Figure 2 shows the relationship between the ND filter number and the power of laser beam irradiated on samples. The laser beam power could be widely changed between OD1 and OD3. The ND filters OD1 (attenuation rate 0.1), OD2 (0.01), and OD3 (0.001) were used in the experiment, since the other filters gave too strong or too weak laser beam energies. The excitation laser beam power density could be varied from about 0.53 to 11800 mW/mm2 using the above ND filters and by controlling the irradiation area of the laser beams and the defocus value from 0 to 100 μm as shown in Figure 3. The defocus value is defined as the distance from actual image plane and was set to be positive as the distance between the objective lens and the sample surface decreased. The places of C60NWs exposed to the excitation laser beams can be recognized as the green circular areas marked in Figures 3(a)–3(f). The area of laser beam on the samples could be changed from 63.8 to 9270 μm2 by controlling the defocus value from 0 to 100 μm.

101243.fig.002
Figure 2: Relationship between the neutral density (ND) filter number and the laser beam power.
101243.fig.003
Figure 3: Optical microscopy images of the samples of C60NWs irradiated by the excitation laser beams for the defocus values (under focus) of (a) 100, (b) 80, (c) 60, (d) 40, (e) 20, and (f) 0 μm and for the arrowed exposed areas of (a) 9270, (b) 6630, (c) 3480, (d) 1470, (e) 617, and (f) 63.8 μm2, respectively. Graph (g) shows the relationship between the defocus value and the exposed area.

The exposed area (𝑦, μm2) and the defocus value (𝑥, μm) were plotted as shown in Figure 3(g). The plotted points can be approximated by the fitted quadratic curve, 𝑦=0.88𝑥2+6.8𝑥+36. Figure 4 summarizes the relationship among the laser beam power density, ND filter number, and the defocus value.

101243.fig.004
Figure 4: Power density of the Raman excitation laser beam measured as a function of ND filter number and the defocus value.

Figure 5 shows examples of the Raman spectra of C60NWs taken by using the ND filters of OD1, OD2, and OD3 for an exposure time of about 220 s, where the spot size of laser beam on samples was 9 μm in diameter. Each power density of the excitation laser beam was (a) 11800, (b) 1660, and (c) 71.5 mW/mm2, respectively. The Ag(2) peak around 1468 cm−1 sifted to the lower wavenumbers with increasing the laser beam power density.

101243.fig.005
Figure 5: Raman spectra of C60NWs. The power density of laser beam (𝐷) is (a) 11800, (b) 1660, and (c) 71.5 mW/mm2, respectively.

Figure 6 shows the Ag(2) peak positions of the Raman spectra of C60NWs as a function of energy dose of the laser beam for each defocus value from 100 μm to 0 μm (just focus). The power density of laser beam on samples was changed by changing the defocus value and the ND filter number as shown in Figure 4. The energy dose was changed by setting the beam exposure time at 215 ± 6 s, 441 ± 10 s, 665 ± 9 s, and 899 ± 29 s for each power density. Hence, as a whole, 72 data points are plotted in Figure 6. As shown in Figure 5, the Raman shifts are found to generally decrease to the lower values with increasing the energy dose. However, the Raman shifts were observed to increase along the red arrows for the high energy doses in Figures 6(c), 6(d), 6(e), and 6(f). These phenomena are supposed to be explained by the temperature rise of the C60NWs exposed to the laser beams, since it is known that the photopolymerized C60 molecules decompose into their primary monomers and dimers by heating at temperatures higher than about 100°C [13].

fig6
Figure 6: Ag(2) peak positions of the Raman spectra of C60NWs under various exposure conditions at the defocus values of (a) 100 μm, (b) 80 μm, (c) 60 μm, (d) 40 μm, (e) 20 μm, and (f) 0 μm (just focus), corresponding to (a) ~ (f) of Figure 3.

The data points obtained using the highest power densities are indicated in each graph of Figure 6 by the black arrows for the exposure time of about 220 s. Figure 7 shows the relationship between the laser beam energy dose and the Ag(2) peak position for the arrowed data points of Figure 6. The fitted curve of semilog plot is expressed as 𝑦=2.2𝑥+1467, where 𝑥 represents log10 (laser beam energy dose) and 𝑦 represents the Raman shift of Ag(2) peak. Using this experimental formula, the energy dose more than about 1520 J/mm2 is found to be necessary for the photopolymerization of C60NWs in air, when the laser light with a wavelength of 532 nm is used.

101243.fig.007
Figure 7: Relationship between the Raman shift of Ag(2) peak and the energy dose of C60NWs irradiated by the excitation laser beams.

Since it is known that the photopolymerization of C60 progresses through the formation of four-membered rings between adjacent C60 molecules [11], it is considered that C60 molecules are linearly polymerized by forming the four-membered rings along the growth axis of C60NWs, as was shown in Figure 6 of [2].

In the gas chromatography-mass spectrometry (GC-MS) measurement of solvents contained in the C60NWs that were prepared by use of toluene and IPA, the major residual solvent was toluene, and the content of IPA was very small compared with toluene [14]. Since the residual toluene of C60NWs was measured to be about 0.2% after drying in an Ar atmosphere at 100°C for 30 min. [14], it is considered that the residual toluene of the vacuum-dried samples of C60NWs in the present experiment is negligible and does not influence the Raman profiles.

4. Conclusions

The photopolymerization of C60NWs was investigated by using the Raman laser beam of 532 nm wavelength at various exposure conditions for the power density and the exposure time in air.

The Ag(2) peak of C60NWs shifted to the lower wavenumbers from that of the as-grown dried C60NWs. However, the Ag(2) peaks were found to move to the higher wavenumbers from the polymerized positions by the irradiation of laser beams for high energy doses at high-power densities, indicating the thermal dissociation of polymerized C60 molecules owing to the temperature rise.

An energy dose larger than about 1520 J/mm2 was found to be necessary for the laser beam of 532 nm wavelength to obtain the photopolymerized C60NWs.

Acknowledgment

Part of this research was supported by Health and Labour Sciences Research Grants (H21-Chemistry-Ippan-008) from the Ministry of Health, Labour, and Welfare of Japan.

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